Laser refractor

ABSTRACT

A laser refractor which utilizes the diffuse reflection of laser light from the interior surface of a slowly rotating drum in order to produce an interference pattern whose apparent motion is a function of the refractive state of the eye.

United States Patent [1 1 Wittenberg [451 Feb. 19, 1974 LASER REFRACTOR[75] Inventor: Sidney Wittenberg, Sudbury, Mass.

[73] Assignee: The Massachusetts College of Optometry, Boston, Mass.

[22] Filed: Nov. 7, 1972 [21] Appl. No.: 304,472

[52] US. Cl 351/36, 351/27, 351/28 [51] Int. Cl A61b 3/02 [58] Field ofSearch 351/36, 27, 28, 30, 17

[56] References Cited UNITED STATES PATENTS 3/1971 Knoll 351/36 OTHERPUBLICATIONS Proceedings of the IRE, Rigden et al. The Granularity ofScattered Optical Maser Light, Vol. 50, Nov. 1962, pp. 2367-2368.

Proceedings of the IEEE, B. M. Oliver, Sparkling Spots & RandomDiffraction, Vol. 51, Jan. 1963, pp. 220 & 221.

Primary Examiner-James W. Lawrence" Assistant ExaminerB. C. AndersonAttorney, Agent, or Firm-Chittick, Thompson & Pfund [57] ABSTRACT A la sHFefraaor vvhic li iifiiiies the omis'e reflection of laser light fromthe interior surface of a slowly rotating drum in order to produce aninterference pattern whose apparent motion is a function of therefractive state of the eye.

13 Claims, 2 Drawing Figures LASER REFRACTOR BACKGROUND OF THE INVENTIONThe present invention relates to ophthalmic instruments in general and,more particularly, to a laser refractor.

Laser refraction has been used experimentally to subjectively determinerefractive status of adults and children. The determination of therefractive state depends upon the detection of motion of a spatialinterference pattern.

The patterns motion results from that of a slowly rotating drum. Theinterference pattern moves in opposite directions on opposite sides ofthe drum and is virtually stationary at the drum. The precision ofmeasurement is ultimately limited by the ability of the subject toobserve the granularity of the spatial interference pattern and todiscern its motion, since the pattern becomes smaller and moves moreslowly upon approaching conjugacy. The motion of the drum is magnifiedas a function of the distance of the interference pattern from the drumso that the motion slows monotonically until at conjugacy it moves withthe velocity of the drum.

The sensitivity of the system is directly related to the rate ofrotation of the drum. By speeding up the drum, the sensitivity aroundneutrality can be greatly increased.

The relative motion of the spatial interference pattern is not only afunction of the rate of drum rotation, but also of the radius of thedrum as well. Decreasing the radius increases the apparent speed of thepattern while at the same time decreasing the size of the elements inthe granular interference pattern.

The apparatus currently in general use for laser refraction comprises acylindrical drum having a diffuse reflecting surface which revolvesslowly about its axis. The drum can also be rotated by means of a gimbalmount about an axis that is perpendicular to the axis revolution. Thedrum is illuminated by a helium neon laser through a beam expander whichincreases the area of the drum which can be illuminated by the otherwisevery narrow laser beam.

Most commonly, the axis of astigmatism is established first by means ofan astigmatic fan dial. The laser drum is rotated about its gimbal mountuntil the rotation of the drum is aligned with the axis indicated by theastigmatic fan chart. With a meridian established, the subject is askedto report the apparent direction of motion of the speckled pattern.Spherical lenses are changed to slow and eventually stop the motion. Thegimbal mount is rotated through 90 and the measurement repeated, usuallywith spherical lenses. A representative example of such a laserrefractor is disclosed in U.S. Pat. No. 3,572,912 issued Mar. 30, 1971to Henry A. Knoll for METHOD OF AND APPARATUS FOR TESTING AMETROPIA BYLASER REFRAC- TION.

A number of limitationsare inherent in the instrumentation of the gimbalmounted drum laser refractor and in the measurement techniques requiredby such instrumentation. First, there is a potential hazard foraccidental direct exposure since the laser beam is projected across anopen room. Secondly, the visual task for the patient is meaningless. Thespeckled pattern is a constantly changing one and it forms the focus ofattention for the task while itself being undefined since the eye willsee perfectly focused patterns regardless of the state of accommodationor refraction. Furthermore, the pattern increases in size with anincrease in defocus of the reflecting plane. As a result, the patientmay very well exert some accommodation. Finally, chromatic aberrationwhich may induce practical differences between the laser determinationof refraction and the routine clinical determination of refraction havenot been dealt with.

The eye also can accommodate even in the presence of excess minus lensesto produce against motion. This is particularly meaningful in light ofthe fact that the patient may very well exert some accommodation.

Finally, there is some experimental data that the determination of thecylinder axis seems to be the limiting constraint in laser refractionalthough no reports of the limits of precision or comparisons withsubjective refraction have been published to date.

Given, among others, the preceding limitations of existing laserrefractors, the parameters of a desirable laser refractor can bedefined. These parameters. are as follows: (1) the laser light sourceshould be contained in or directly attachable to the chamber of therotating drum; (2) accommodation should be controlled by providing adefinite fixation for the subject which is independent of the laserpattern; (3) calibration to compensate for chromatic aberration isdesirable; (4) minimization of the effect of the laser pattern on thecharacter of the measurement (the pattern should not affectaccommodation); and, (5) the astigmatic axis and manner of astigmatismshould be established by means of the characteristics of the speckledpattern itself. Preferably, the laser refractor should be able tomeasure an eye monocularly under binocular viewing and have the abilityto measure characteristics of accommodation.

It is, accordingly, a general object of the present invention to providean improved laser refractor which fulfills the parameters set forthabove.

It is a specific object of the present invention to provide a laserrefractor which accomplishes the general object of the invention byutilizing readily available, optical, electro-optical and mechanicalcomponents.

It is a feature of the present invention that the various optical,electro-optical and mechanical components of the laser refractor can beassembled in a compact and self-contained instrumentation package. I

These objects and features and other objects of th present inventionwill best be understood from a detailed description of a preferredembodiment thereof, selected for purposes of illustration and shown inthe accompanying drawings, in which:

' FIG. 1 is a perspective view of the laser refractor of the presentinvention showing portions thereof broken away for purposes of clarity;and,

FIG. 2 is a diagrammatic view showing the optical path of the laserrefractor of the present invention.

Turning now to the drawings, there is shown an improved laser refractorconstructed in accordance with the present invention and indicatedgenerally by the reference numeral 10. The desired fixation andaccommodative control for'a patient or subject-12 are provided by atarget slide (not shown) which is projected by projector 14 onto ascreen 16. The screen is located at a practical testing distance, e.g.20 feet, from the projector andsubject.

The fixation target shown by an arrow 18 on the screen is viewed by thesubject through a beam splitter, dichroic mirror or pellicle mirror 20.The beam splitter 20 superimposes a view of the interior of a slowlyrotating, laser refractor drum 22 on the screen. The hollow refractordrum 22 has a diffuse, light reflecting inner surface 24 which may beeither an integral part of the drum or formed as a separate coating orremovable sheet. An axially adjustable, positive calibration lens 26images the internal reflecting surface of the drum at any desiredposition in space. Normally, the surface is imaged near infinity. Avariety of drum geometries can be employed in practicing the invention.For example, the drum can be cylindrical, toroidal, or spherical. In thepreferred embodiments, a right circular cylindrical configuration isused for the refractor drum 22.

The internal surface of the drum is illuminated by a monochromatic,coherent light generated by a light source, such as laser 28. The laser28 generates a very narrow beam 30 of coherent light which passesthrough an optical shutter system 32 and is expanded by means of a beamexpander 34. The beam expander is used only if the cross-sectional areaof the light beam illuminates an area on the drum surface which isinsufficient to fill the field of view of the observer.

The expanded laser light 36 enters the laser refractor drum 22 along theaxis thereof and is reflected by a mirror 38 onto the interiorreflecting surface 24 of the drum. The expanded laser light reflectedback from the interior surface of drum 22 forms the spatial interferencepattern which, after reflection from a second mirror 40 which directsthe light out of the drum 22 through the previously mentionedcalibration lens 26 and to the beam splitter 20, is viewed by thesubject being measured.

The laser refractor drum 22 is slowly rotated by means of a drive systemindicated generally by the reference numeral 42. The drive system 42comprises a motor 44 and two support rolls 46 and 48. Motive power frommotor 44 is transmitted to support roll 46 through a power transmissionbelt 50. The source of electrical power for motor 44 and the controlcircuitry associated therewith have been omitted from the drawings forpurposes of clarity and need not be discussed in any detail because theyare well known to those skilled in the art.

The calibration lens 26 and the mirrors 38 and 40 are mounted within anaxially rotatable housing 52 which is supported by a bearing mount 54.Rotation of the mirror housing 52 is used to change the virtual(optically reduced) meridian of motion of the rotating drum 22. Asuitable scale 56 is secured to and rotates with the housing 52. Thescale 56 is indexed by means of a fixed index 58 to indicate themeridian of apparent drum rotation.

With the drum rotating slowly in the direction shown by the arrow inFIG. 1, the subject views the interior reflecting surface of the drumthrough lens 26. Both of the subjects eyes are left open but only oneeye views the spatial interference pattern and refers it to the screen16. The eye not being measured is fogged slightly. Filters 60 and 62,which may be fixed or adjustable, are used to more nearly equalize theintensity of the images in the two eyes and to control the relativeapparent brightness of the observed interference pattern. A slightfogging lens (not shown) may be placed between the beam splitter and thescreen thus effecting the clarity of vision without effecting themeasurement. Although the pattern may be viewed continuously during theearly stages of measurement, the shutter 32 is normally used in thefinal determination to insure minimum effect of the pattern in themeasurement.

The axis of astigmatism is determined by having the subject point thetarget 18 in the direction of motion of the pattern. lf the alignment ofthe target and the setting of scale 56 with respect to the fixed index58 do not agree, the housing 52 is rotated appropriately and the settingrepeated. It will be appreciated that this technique is similar to thatfor determining cylinder axis retinoscopically and converges quickly onthe correct axis within three or four trials. The amount of astigmatismis measured by using cylindrical lenses to stop the motion of theinterference pattern.

For the measurement of refractive error the pattern can also be stoppedby moving the mirrors 38 and 40 axially within the housing 52. Thismovement is indicated in FIG. 2 by the two-way arrow through mirrors 38and 40. By moving these two mirrors as a unit, the light spot 64 ismoved along the inner reflecting surface 25 parallel to the axis of thedrum with no apparent movement created in the visual field in a planenormal to the axis. This movement changes the distance between the spot64 and the axis lens 26 and effectively changes the power of the system.Thus, by calibrating the axial movement of two lenses as a unit, it ispossible without supplementary lenses to determine the power to correctthe eye. The same technique also can be employed by moving the positivecalibration lens 26 along the drum axis as shown by the arrows in thediagrammatic view of FIG. 2.

From the preceding description it can be appreciated that the laserrefractor of the present invention provides for the measurement of theaxis and amount of astigmatism through the use of an internallyilluminated slowly revolving drum and that the measurement can be madewith respect to any plane since an external fixation target can be used.Furthermore, the motion of the drum can be referred to any meridian ofthe eye, measurements can be made under either monocular or binocularfixation and accommodation as well as refractive state can be measured.

Having described in detail a preferred embodiment of my invention, itwill be appreciated that numerous variations can be made therein withoutdeparting from the scope of the invention as defined in the appendedclaims. For example, the functions of the beam expander 34 and firstlight directing mirror 38 can be combined in a single element in theform of a spherical mirror having a relatively short radius. Thespherical mirror is positioned slightly off axis in the general locationof the mirror 38. The use of a spherical mirror, both as a beam expanderand a light director, eliminates the need for a separate beam expander34 and a first light directing mirror 38, thereby producing a morecompact system. I

Additionally, two different wavelengths of monochromatic coherent lightcan be used to bracket both sides of the peak spectrum. In thissituation, two different wavelength lasers are employed with suitablemeans, such as a beam splitter for directing both beams of laser lightinto the drum 22 along the axis thereof.

What I claim and desire to secure by letters Patent of the United Statesis:

l. A refractor comprising:

1 drum means having a diffuse inner light reflecting surface;

2 means for rotating said drum means about its axis;

3 means for generating at least one beam of monochromatic coherentlight;

4 means for directing the light beam to the diffuse light reflectinginner surface of said drum means;

5 means for moving said light beam along the diffuse light reflectinginner surface of the drum means in a plane which is normal to the axisof the drum means; and,

6 optical means for observing the illuminated portion of the diffuseinner light reflecting surface of the drum.

2. The apparatus of claim 1 wherein said means for generating a beam ofmonochromatic light comprises a laser.

3. The apparatus of claim 1 wherein said means for directing the lightbeam to the diffuse light reflecting inner surface of the drum comprisesa spherical mirror positioned within and off the axis of said drum.

4. The apparatus of claim 1 wherein said means for directing the lightbeam to the diffuse light reflecting inner surface of the drum comprisesa planar mirror positioned within the drum means along the axis thereof.

5. The apparatus of claim 4 further comprising means for expanding thelight beam with the expanded light beam being directed along the axis ofthe drum to impinge upon said planar mirror.

6. The apparatus of claim 1 further comprising index means forindicating the meridian of virtual drum rotation.

7. The apparatus of claim 1 further characterized by means forcontrolling the amount of light which is directed into the subjects eye.

8. The apparatus of claim 1 further comprising shutter means forcontrolling the duration of the light which is directed into thesubjects eye.

9. A refractor comprising:

1 drum means having a diffuse inner light reflecting surface;

2 means for rotating said drum means about its axis;

3 means for generating at east one beam of monochromatic coherent light;

4 means for directing the light beam to the diffuse light reflectinginner surface of said drum;

5 reflected light directing means positioned within said drum meansalong the axis thereof for intercepting the light reflected from thediffuse reflecting inner surface of the drum means, said light directingmeans directing the reflected light out of said drum means along theaxis thereof; and, 6 means for rotating together said light beam direct-5 ing means and said reflected light directing means about the axis ofsaid drum means.

10. The apparatus of claim 9 further comprising axially adjustable,positive lens means positioned along the axis of the drum to interceptthe reflected light which is directed out of said drum means along theaxis thereof.

11. The apparatus of claim 10 further comprising means for axiallymoving together said light beam directing means and said reflected lightdirecting means along the axis of said drum means.

12. A laser refractor comprising:

1 cylindrical drum means having a diffuse light reflecting innersurface;

2 means for rotating said cylindrical drum means about its axis;

3 laser means for generating a beam of coherent light;

4 means for expanding said coherent light beam with the expanded beam ofcoherent light being directed into said cylindrical drum means along theaxis thereof;

5 first light directing means positioned within said cylindrical drummeans along the axis thereof for directing the expanded coherent lightbeam to the diffuse light reflecting inner surface of said cylindricaldrum;

6 second light directing means positioned within said cylindrical drummeans along the axis thereof for intercepting the light reflected fromthe diffuse reflecting inner surface of the cylindrical drum means, saidsecond light directing means directing the reflected light out of saidcylindrical drum means along the axis thereof;

7 means for rotating together said first and second light directingmeans about the axis of said cylindrical drum means; and,

8 means positioned outside of said cylindrical drum means for directingsaid reflected light into a subjects eye.

13. The apparatus of claim 12 wherein said first light

1. A refractor comprising: 1 drum means having a diffuse inner lightreflecting surface; 2 means for rotating said drum means about its axis;3 means for generating at least one beam of monochromatic coherentlight; 4 means for directing the light beam to the diffuse lightreflecting inner surface of said drum means; 5 means for moving saidlight beam along the diffuse light reflecting inner surface of the drummeans in a plane which is normal to the axis of the drum means; and, 6optical means for observing the illuminated portion of the diffuse innerlight reflecting surface of the drum.
 2. The apparatus of claim 1wherein said means for generating a beam of monochromatic lightcomprises a laser.
 3. The apparatus of claim 1 wherein said means fordirecting the light beam to the diffuse light reflecting inner surfaceof the drum comprises a spherical mirror positioned within and off theaxis of said drum.
 4. The apparatus of claim 1 wherein said means fordirecting the light beam to the diffuse light reflecting inner surfaceof the drum comprises a planar mirror positioned within the drum meansalong the axis thereof.
 5. The apparatus of claim 4 further comprisingmeans for expanding the light beam with the expanded light beam beingdirected along the axis of the drum to impinge upon said planar mirror.6. The apparatus of claim 1 further comprising index Means forindicating the meridian of virtual drum rotation.
 7. The apparatus ofclaim 1 further characterized by means for controlling the amount oflight which is directed into the subject''s eye.
 8. The apparatus ofclaim 1 further comprising shutter means for controlling the duration ofthe light which is directed into the subject''s eye.
 9. A refractorcomprising: 1 drum means having a diffuse inner light reflectingsurface; 2 means for rotating said drum means about its axis; 3 meansfor generating at least one beam of monochromatic coherent light; 4means for directing the light beam to the diffuse light reflecting innersurface of said drum; 5 reflected light directing means positionedwithin said drum means along the axis thereof for intercepting the lightreflected from the diffuse reflecting inner surface of the drum means,said light directing means directing the reflected light out of saiddrum means along the axis thereof; and, 6 means for rotating togethersaid light beam directing means and said reflected light directing meansabout the axis of said drum means.
 10. The apparatus of claim 9 furthercomprising axially adjustable, positive lens means positioned along theaxis of the drum to intercept the reflected light which is directed outof said drum means along the axis thereof.
 11. The apparatus of claim 10further comprising means for axially moving together said light beamdirecting means and said reflected light directing means along the axisof said drum means.
 12. A laser refractor comprising: 1 cylindrical drummeans having a diffuse light reflecting inner surface; 2 means forrotating said cylindrical drum means about its axis; 3 laser means forgenerating a beam of coherent light; 4 means for expanding said coherentlight beam with the expanded beam of coherent light being directed intosaid cylindrical drum means along the axis thereof; 5 first lightdirecting means positioned within said cylindrical drum means along theaxis thereof for directing the expanded coherent light beam to thediffuse light reflecting inner surface of said cylindrical drum; 6second light directing means positioned within said cylindrical drummeans along the axis thereof for intercepting the light reflected fromthe diffuse reflecting inner surface of the cylindrical drum means, saidsecond light directing means directing the reflected light out of saidcylindrical drum means along the axis thereof; 7 means for rotatingtogether said first and second light directing means about the axis ofsaid cylindrical drum means; and, 8 means positioned outside of saidcylindrical drum means for directing said reflected light into asubject''s eye.
 13. The apparatus of claim 12 wherein said first lightdirecting means comprises a planar mirror positioned at an obtuse anglewith respect to the axis of said cylindrical drum means and wherein saidsecond light directing means comprises a planar mirror positioned at anangle of 45* with respect to said axis so that a plane defined by thenormals of the two mirrors also contains said drum axis.